A conventional pressure-type flow rate control apparatus, for example, is configured by coupling a main block 1 that has main flow passages 1a and 1b, an inlet-side block 2 that has an inlet-side flow passage 2a, and an outlet-side block 3 that has an outlet-side flow passage 3a together so that the flow passages 2a, 1a, 1b, and 3a communicate with one another, as illustrated in FIG. 8. A valve body 4, such as a metal diaphragm valve, that is provided between the main flow passages 1a and 1b is openable and closable by a piezoelectric actuator 5 that is attached to the main block 1 (see Patent Document 1).
Known structures include a structure in which a gasket-type orifice 7 with an orifice plate 6 fixed thereto for use in flow rate control is inserted between the outlet-side block 3 and the main block 1 (Patent Document 1), and a structure in which a filter gasket 9 with a filter plate 8 fixed thereto is inserted between the inlet-side block 2 and the main block 1 (e.g., Patent Documents 1 to 4).
In this type of pressure-type flow rate control apparatus 10, when a so-called critical expansion condition of (P1/P2)≥approx. 2 holds between a downstream pressure P2 and an upstream pressure P1 of the orifice plate 6, the flow rate Q of a gas flowing through the orifice of the orifice plate 6 is given by the relationship Q=KP1 (where K is a constant).
Using such a relationship enables high-precision control of the flow rate Q by controlling the pressure P1 detected by a pressure sensor 11, and achieves such excellent characteristics that even when the pressure of a gas G0 on the upstream side of the valve body 4 changes greatly, the controlled flow rate value hardly changes.
The orifice plate or the filter plate is typically fixed by welding to an orifice base or a filter base. In the case of a sintered filter plate that cannot be welded, a method is also known, in which an annular lip is provided at, for example, the filter base and bent (caulked) inward to fix the filter plate (e.g., Patent Document 3). There is also a method in which the orifice base or the filter base is divided into halves that are able to fit together, and when both halves are fitted together, the orifice plate or the filter plate is inserted between the halves (e.g., Patent Documents 1, 2, and 4). In the case of using such a halved base, typically the orifice plate or the filter plate is laser-welded to one half of the orifice base.